The present invention relates generally to a gas turbine engine airfoil and a method of fabrication thereof. More particularly, the present invention has one embodiment wherein the airfoil includes a preformed cast cover member metalurgically bonded to an inner structural spar member. Although the invention was developed for gas turbine engines, certain applications may be outside of this field.
A gas turbine engine is typical of the type of turbo machinery in which the invention described herein may be advantageously employed. It is well known that a conventional gas turbine engine comprises a compressor for compressing air to an appropriate pressure necessary to support the combustion of a fuel in a combustion chamber. The high temperature exhaust gas exiting the combustion chamber provides the working fluid for a turbine, which powers the compressor. A power turbine driven by the flow of high temperature gas is utilized to turn a propeller, fan or other propulsion device. Further, the high temperature gas may be used directly as a thrust for providing motive power, such as in a turbine jet engine.
It is well known that the performance of gas turbine engines increases with the increase in the operating temperature of the high temperature gas flowing from the combustion chamber. A factor recognized by gas turbine engine designers as limiting the allowable temperature of the gaseous working fluid flowing from the combustion chamber is the capability of the various engine components to not degrade when exposed to the high temperature gas flow. Further, gas turbine engine designers are fully cognizant that the engine's airfoils are among the components exposed to the maximum thermal and kinetic loading during engine operation.
A variety of techniques have been integrated into the gas turbine engine blades and vanes to minimize their degradation as they are exposed to the high temperature gases. Film cooling, a standard technique generally integrated into blade and vane design, refers to a technique of cooling an external surface of the component by injecting a relatively cool media, such as air along the component's external surface. The cooling media functions as an insulating layer to reduce the unwanted heating of the external surface of the component by the flow of high temperature gas.
A second conventional technique often incorporated into the component design is an internal network of apertures and passageways within the component. A steady flow of pressurized cooling media is passed through the internal passageways of the component, and the cooling media is finally exhausted through the apertures onto the exterior surface of the component. The passage of the cooling media through the internal passageways and out through the exit apertures provides for convective heat transfer from the component walls to the cooling media.
Many prior gas turbine engine airfoils, such as vanes and blades, have been produced by production techniques using labor intensive complicated casting methods or laminating procedures. The typical lamination process utilized to produce an airfoil involves the high temperature diffusion or braze bonding of multiple layers of wrought material together to form the airfoil. Casting a sophisticated airfoil has required a generally elaborate procedure often with a relatively low yield that tended to be very labor intensive.
Although the prior techniques have produced airfoil components with internal passageways, exit apertures and film cooling, the need remains for an improved method and apparatus for making a fabricated gas turbine engine airfoil with a cover sheet metalurgically bonded to a spar. The present invention satisfies this need in a novel and unobvious way.